Patent document 1 describes an abnormality diagnosis such as a primary check in an air bag sensor of a vehicular occupant protection system. Here, pulse signals (diagnosis signals, abnormality-diagnosis signals) with given intervals are inputted to the air bag sensor, so that an abnormality diagnosis is executed based on output signals from the air bag sensor. However, when an input terminal for the abnormality-diagnosis signals experiences electrical noises, the air bag sensor sometimes involves mis-operation. That is, an abnormality diagnosis for the air bag sensor cannot be sometimes accurately performed.
On the other hand, a vehicular occupant protection system includes multiple sensor sub-systems (or sensor units) including air bag sensors such as satellite acceleration sensors in various portions in a vehicle. Further, the vehicular occupant protection system includes in a center portion in the vehicle a main control unit that determines abnormality in the sensor sub-systems while controlling ignition of squibs. Here, in a conventional vehicular occupant protection system, the main control unit and each of the sensor sub-systems are directly connected with each other.
However, as the number of sensor sub-systems is recently increased, the connection between the main control unit and each of the sensor sub-systems comes to be achieved by using a single bus (refer to Patent documents 2, 3). For instance, the number of interfaces of the main control unit can be thereby decreased to thereby reduce a size; and further, the number of sensor sub-systems can be easily changed in designing.
In the above conventional vehicular occupant protection system connected using the bus, when abnormality diagnosis such as a primary check is executed, the following processes take place. Here, each of the sensor sub-systems includes an air bag sensor and an abnormality diagnosing unit. In the primary check, the main control unit outputs an abnormality-diagnosis start command to an abnormality diagnosing unit of a first sensor sub-system because of an ignition switch turning on. The abnormality diagnosing unit of the first sensor sub-system then starts an abnormality diagnosis for the corresponding air bag sensor to then generate an abnormality-diagnosis result. The abnormality diagnosing unit of the first sensor sub-system then outputs the abnormality-diagnosis result to the main control unit. The main control unit subsequently outputs an abnormality-diagnosis start command to an abnormality diagnosing unit of a second sensor sub-system. Likewise, the abnormality diagnosing unit of the second sensor sub-system then starts an abnormality diagnosis for the corresponding air bag sensor to then generate an abnormality-diagnosis result. The abnormality diagnosing unit of the second sensor sub-system then outputs the abnormality-diagnosis result to the main control unit. This process is repeated to all sub-systems, thereafter terminating the abnormality diagnosis in the system.
Thus, in the abnormality diagnosis such as the primary check in the vehicular occupant protection system, after the abnormality diagnosis of the first sensor sub-system is terminated, the abnormality diagnosis of the second sensor sub-system is started. That is, as the number of sub-systems increases in the conventional vehicular occupant protection system, a time period required for the primary check increases.                Patent document 1: JP 2001-91536 A        Patent document 2: JP 2003-285716 A (U.S. 2004/0104562 A1, U.S. 2004/0103811 A1, U.S. 2004/0084882 A1)        Patent document 3: JP 2001-322527 A        